Distribution and palaeoecological indicator value of diatoms in Holocene coastal deposits of the southern North Sea (Lower Saxony, Germany)

Abstract

The development of the North Sea coastal landscapes during the past 8000 years has been strongly influenced by sea-level fluctuations that involved cyclical transgression, stagnation and even regression phases. Transgressive phases are reflected in NW Germany by Holocene coastal deposits consisting mainly of sand, silt and clay, whereas phases of regressive coastal development are indicated by the presence of peat layers. Although the East Friesian barrier islands in their present state are not older than 2000 years, they possess material that is amenable to palaeoecological investigations. Fossil salt marsh and tidal flat horizons provide natural sea-level indicators that record fluctuations of the mean high tide level in the southern North Sea. This paper presents the results of diatom analyses from boreholes in coastal regions and on the East Friesian barrier islands. Diatom assemblages of the clastic units of marine transgression phases are generally characterized by marine pelagic and littoral diatoms. Nevertheless, oligo- and mesohalobous species also occur, predominantly in direct contact with the basal peat layer suggesting that the first marine transgression proceeded slowly. Tidal flat deposits are characterized by poly- and mesohalobous species, whereas lagoonal sediments are dominated by littoral brackish and sometimes fragile diatoms indicating low salinity and calm depositional conditions. Intercalated Phragmites fen peat layers show an increase in littoral oligohalobous species reflecting regressive phases of sea level. Salt-marsh deposits on the East Friesian islands, as well as those in contact with the Pleistocene hinterland, mainly contain aerophilous, littoral species. Flooding events and increasing marine influences are documented by higher amounts of pelagic marine diatoms. Dune slack deposits, which only occur on the islands, and deposits in the uppermost salt-marsh areas show a clear dominance of oligohalobous species suggesting that marine influences are restricted in those environments.